1. Field of the Invention
A synchronous rectifier with dead-time adjusting function especially relates to a synchronous rectifier using fly-back power supply.
2. Description of Related Art
Most of prior art direct current power supplies, for example, AC to DC switching power supply, use high-frequency pulse width modulation (PWM) for shrinking the volume of a transformer. FIG. 1 shows a circuit schematic diagram of a prior art fly-back power supply. The circuit of a transformer T1 is divided into a front-stage circuit 101 at a primary side and a back-stage circuit 102 at a secondary side. Optical signals transferred between the primary side and the secondary side are separated by a photo transistor 111 and a photodiode 112, and changes of the current and the voltage of the secondary side can be fed back to the primary side by optical signals. Therefore, the changes of the current and the voltage in the primary side 101 and the secondary side 102 may be adjusted synchronously and the fed back signals can provide overcurrent protection and short protection.
Fly-back transformers include a circuit for transforming alternative current to direct current, and working principle thereof is: when a transistor Q1 is on, energy is stored in a transformer T1 because poles of primary side 101 are opposite the poles of the secondary side 102 and a transistor Q2 is off; after the poles of the transformer T1 are inverted, the transistor Q2 will be on and the energy stored in the transformer T1 will be released.
Dead time occurs when the transistor Q1 and the transistor Q2 are both off during a period of time. In prior art fly-back power supply, the duty cycle of the pulse width modulation for controlling DC output voltage is not fixed and it may worsen the use rate of the transistor Q2.
Delay occurs when the transistor Q1 and the transistor Q2 switch states; hence, the transistor Q1 and the transistor Q2 are both on temporarily at the same time, and it will cause the switching loss of the circuit. The switching loss will be more serious when the circuit is operated with high output voltage, and the transistor Q2 will be burned up easily.
Taiwan patent No. 519789 discloses a fly-back type power supply with synchronous rectifying function, in which a current comparator is connected to the secondary side of a transformer for acquiring the output loading current. A comparator is used for comparing the current and driving synchronous switch. Dead time is not stable and changes with the variation of the supply voltage of the primary side in the above-mentioned fly-back type power supply. The dead time lasts longer when the supply voltage is low; therefore, the utilization rate of the synchronous switch is worse. Because the current comparator is made of an iron core and a coil, it cannot be used in integrated circuits.